It is an important aspect in catalytic reactions employing catalysts which have to be replaced periodically, e.g. for regeneration, to reduce the off-stream time of the reactors as much as possible. It has been suggested to use swing reactor systems which allow production to continue in one or more reactors while the catalytic material present in one or more other reactors is being replaced or regenerated. These systems, however, give rise to relatively large and complicated plants and are therefore expensive.
A major improvement has been described in British Patent Specification No. 1,384,762 which refers to a continuous process for the catalytic treatment of hydrocarbon oils which is carried out in one or more reactors to which fresh catalyst is added and spent catalyst is withdrawn (either periodically or continuously) during operation. In this way it is possible to maintain a continuous catalytic process having moreover a substantially constant activity. Liquid and gaseous hydrocarbon oils are conveniently separated from spent catalyst in the process described in British Pat. No. 1,384,762 via a discharge system present in the bottom of reactor. The discharge funnel incorporates a screen which separates the hydrocarbon oil and the product(s) obtained by the catalytic treatment of the hydrocarbon oil from the catalyst applied, said screen being located upstream of the exit of said funnel. The spent catalyst is withdrawn through the discharge funnel and the separated liquid and gaseous materials are withdrawn through a product outlet in the bottom of the reactor.
The catalyst discharge funnel preferably comprises at least two inverted conical sections the bottom of an upper section being connected to the top of a lower section by means of a cylindrical screen section. The conical sections facilitate the mass-flow of catalyst down the reactor. The walls of the screen are normally parallel to the direction of catalyst flow, viz. at 90.degree. to the horiozontal plane. This arrangement is favorable because the resistance to catalyst flow is minimized. This point may be especially important where a woven screen is used which does not favor catalyst flow down the reactor because of its rough nature.
Since the main function of the screen resides in providing separation of liquid and gaseous hydrocarbons from catalyst particles, it will be clear that the openings in the screen are such that catalyst particles cannot pass through the screen.
A problem arises, however, when the catalyst mass travelling through the reactor also contains broken particles (either present originally or originating from catalyst particles or inerts present in the reactor) and/or particles, of a smaller size which may either pass through the screen, or, even worse, tend to get stuck into openings in the screen blocking passage of liquid and gaseous materials through the screen. Also the relatively low pressure differential which is formed advantageously by the normal use of the screen will be disturbed gradually which seriously affects operation efficiency.